The present invention relates generally to a method for agglomerating ceramic materials and, more particularly, to a method for agglomerating boron carbide powders.
Boron carbide (B4C) Is a useful ceramic compound. B4C is third hardest material known, following diamond and cubic boron nitride (cBN). B4C is hard, lightweight, and wear resistant, making it attractive for such applications as sandblasting nozzles, bearings, dies, lapping and polishing abrasive powders, sporting equipment (as part of a metal matrix composite in high-performance bicycles, golf clubs, tennis rackets), and armor (such as armor tiles and bulletproofing materials). B4C is also useful as an anti-oxidant additive to refractory materials, such as in magnesia-carbon bricks. B4C is also useful as a raw material for the production of other boron-containing materials such as titanium boride and boriding agents. B4C may be used as a solid fuel propellant for ducted rockets. Boron carbide containing welding rods are used to improve the wear resistance of welded surfaces. Boron carbide is also a good neutron absorber, having an exceptionally high cross-section for thermal neutrons.
Accordingly, B4C is attractive as a lightweight radiation shielding material. Boron carbide is used both in powder and solid form to control the rate of fission in nuclear reactors. B4C is readily oxidized, and is therefore usually mixed with other structurally attractive materials, such as aluminum metal or polyethylene plastic, to protect it from oxidation in reactor environments. B4C metal-matrix composite (MMC) plates have wide applications as isolators in spent fuel element racks, inner sections of reactor shields, shutdown control rods, neutron curtains, shutters for thermal columns, and shipping containers. B4C metal-matrix composites can withstand temperatures up to about 540xc2x0 C.
B4C may be produced by a number of known methods. One method for producing boron carbide is disclosed in U.S. Pat. No. 2,834,651. The ""651 patent discloses a method of producing fine boron carbide of fine particle size by heating a mixture of boron oxide, carbon and magnesium. While efficient, this process is unsuitable for producing highly pure boron carbide or boron carbide free of magnesium-containing impurities.
Very fine powders of boron carbide have been produced by vapor phase reactions of boron compounds with carbon or hydrocarbons, using laser or plasma energy sources. These reactions tend to form highly reactive amorphous powders. Due to their extreme reactivity, handling in inert atmospheres may be required to avoid excessive oxygen and nitrogen contamination. These very fine powders have extremely low bulk densities which make loading hot press dies and processing greenware very difficult.
Another method known in the art for producing boron carbide powder is described in U.S. Pat. No. 3,379,647. That method involves a carbothermic reduction of boron oxide. According to the ""647 patent, a reactive mixture comprising a carbon source, such as finely divided carbon, and a boron oxide source, such as a boron oxide, is prepared and then fired at a relatively high temperature, whereby the boron oxide which is present initially or which is formed thereupon is reduced, the corresponding boron carbide being concurrently produced. This reaction ordinarily proceeds according to the general equation:
2B2O3+7Cxe2x80xa2B4C+6CO
Generally, the temperature of firing the reactive mixture above is in the range of 1700xc2x0 C.-2100xc2x0 C. The reaction is generally carried out in a protective, non-interfering atmosphere such as an inert gas or a vacuum. Boron carbide is not a congruently melting/fusing material, and so as B4C precipitates from the melt, the remaining melt becomes increasingly rich in carbon. Accordingly, attention must be given to the rate at which B4C is precipitated from the melt, such that the liquid and solid precipitate do not reach an equilibrium. The final composition of the resulting solid material is ideally a B4C phase and a graphite phase. One shortcoming of this method is that substantially all of the B4C produced is not below one micron in size, and a uniform particle size distribution (PSD) is not obtained. Accordingly, the particle size of boron carbide can range anywhere from 0.5 to 150 microns with little control of PSD.
Another known method of producing boron carbide is described in U.S. Pat. No. 4,804,525. The ""525 patent discloses a method of producing boron carbide powder of submicron size by passing a particulate reactive mixture of a boric oxide source and a carbon source through a hot zone such that substantially all of the particles are separately and individually heated at a sufficient temperature and for a sufficient length of time to form boron carbide crystals of submicron size.
While many techniques are known for producing boron carbide, most yield B4C in the form of a powder having a PSD peaking at about 3-5 microns or finer. Moreover, many of the techniques producing larger B4C particles do so with little PSD control and/or little purity control. For many applications, it is desirable to be able to reliably produce larger B4C particles. For example, B4C is a good neutron absorber (having a neutron absorption cross section for thermal neutrons of around 755 barns) and has the advantage of being lightweight. Neutron shielding material can be produced containing B4C suspended in a metal matrix. One commonly selected matrix metal is aluminum, since aluminum is lightweight, strong and relatively inexpensive. Currently, neutron shielding comprising B4C suspended in an aluminum matrix is produced by cold-pressing aluminum powder containing dispersed ultrafine B4C particles into a green body having a desired shape and then sintering the green body at a temperature below the melting point of aluminum (about 660xc2x0 C.). However, this process is relatively expensive and time consuming.
Another technique for producing neutron shielding material from B4C and aluminum is to mix particulate B4C into molten aluminum and cast the resulting melt into bodies having the desired shape. While relatively quick and cheap, this process suffers from the problem that molten aluminum is highly caustic and will readily dissolve B4C, requiring that the B4C particles added to the melt be sufficiently large to survive total dissolution.
One method of reliably producing larger B4C particles would be to agglomerate finer B4C particles into larger ones. Currently, there is no known method of agglomerating pre-existing B4C particles. B4C has a low oxidation temperature of 400-500xc2x0 C., which makes it difficult to thermally process in non-reducing atmospheres.
Prior attempts at agglomerating B4C have typically resulted in the transformation of the starting B4C powder into a puddle of B2O3 glass. There are currently no known techniques for cheaply, quickly and reliably agglomerating B4C into particles having controllable particle size distributions peaking in the 5-20 micron range or larger. There is therefore a need for a technique for rapidly and inexpensively producing B4C agglomerates. The present invention addresses this need.
One form of the present invention relates to a process for producing relatively coarse, agglomerated ceramic particles from relatively fine powder precursors. Another form of the present invention relates to a method for the agglomeration of submicron boron carbide particles, resulting in boron carbide agglomerates in the 5-20 micron size range or larger.
One object of the present invention is to provide an improved method for producing agglomerating ceramic particles. Related objects and advantages will become apparent from the following description.